Precisely, they are 2m high. I've often been on rwys with dew and fog drifting over the grass surrounding the transmittometers. The rwy was crystal clear. When I asked what the RVR was ATC replied the vis was 10k's +, but indeed the RVR was quite low. I've also had an approach ban on a RWY with RVR's 450m. Crossing the OM en-route to the hold we could see the whole rwy, but RVR's are RVR's. Fortunately there was a local CAT 3 operator following us. We set up 4nm behind him and asked for PIREPS. At all time down the G/S we could see the whole rwy. The landing captain reported vis "well in excess of what ATC was giving." We call field in sight and landed, no problems.
There was also an occasion at Luton, when the had electronic RVR's and also the old wooden tower by the threshold. The RVR's were <CAT 1. WE were visual. We asked for a visual RVR from the wooden tower. This confirmed the truth of much higher RVR's at pilot eye height and we landed.
And I'm still sure the definition if required RVR is quoted as being at Pilot eye height and is Xm's above the rwy. IMHO the transmittometers are not fool proof.
And the original question here was about ATC met vis, which when >3km's is surely not an exact science.